to each their own i guess.
my rationale with it was because they are shallow tanks. the footprint of these tanks give it more swim space horizontally than vertically. This is possibly one of the few ways to keep many bettas in the smallest space possible as the OP wanted to ask about. I personally keep a betta in a 2.5 gallon shallow nano tank with no problems...
Of course, if space isn't an isssue then bigger tanks is definitely better for any fish.